Introduction

4B.E.K. Klein

Myopia has been considered to be a problem with origins in childhood. The estimated prevalence in 6-year-olds is 2% and in 15-year-olds, 15%.4 However, adult onset myopia is not an infrequent occurrence. Furthermore, myopic shifts in refraction can occur across the lifespan, although more common in the first two decades than in older persons, and affects those with hypermetropic refractive errors and emmetropes as well as myopes. Thus, this chapter will describe the distribution of myopia and myopic shifts in refraction as reflected in a (non-scientific) sample of studies worldwide in children and adults.

In addition, risk factors that have been evaluated for myopia will be briefly described. The list of risk factors includes a brief description of familiality or family aggregation and in some cases, actual genetic loci.

The reader should be aware that a comprehensive review paper of epidemiology of myopia was published in 1996.5 In addition, a description of the literature on refraction in general was published in 2008.6 The material in this chapter overlaps in part the information in these two sources.

Methodologic Issues

Studies reported upon in this chapter are of several types, including traditional population-based surveys, studies of special exposure groups such as students, occupationally exposed workers, social or ethnic groups, and rural or urban groups. Some studies were performed on convenience samples, although these were largely avoided. In addition, some studies are cross-sectional, while others incorporate longitudinal follow-up. Some case-control studies have been included as well. Risk factors or risk indicators of myopia are briefly discussed. In addition, no survey of risk factors associated with virtually any condition is complete without at least a brief review of genetic correlates of the condition. To that end, this chapter includes some investigations that have approached the study of myopia using the tools of statistical genetics, with data sources from population-based, family, and twin studies, and combinations of these designs. The field of genetics is progressing rapidly and will be further advanced by the time of the publication of this tome. Nevertheless, although the content will be somewhat dated, the approach to investigating these important determinants will still be relevant.

Measurement techniques of refraction varied between studies.7 In most studies in children and in some studies in adults, cycloplegic agents were

5Epidemiology of Myopia

instilled prior to refraction and, among these, different pharmacologic agents were used, likely resulting in some variation in the actual amount of cycloplegia attained. In other studies, no cycloplegic agents were used. The data may have been reported only as continuous data with mean refraction given, while in other cases, the authors reported the data in categories. When the latter was so, the category limits for myopia (and for hyperopia and emmetropia as well) may have differed between studies. Objective refraction was performed in some studies and subjective refraction or refinement in others. In addition, the testing for refraction was based on best corrected distance acuity. This may differ with regard to the use of charts or projection and distances. These details may not have been reported in the publications reviewed. Automated refractors may have been used and these devices differ in design, and consequently could yield systematic differences between results. Lastly, many studies reported the spherical equivalent, while others only gave spherical refraction. All these factors are likely to have led to variation in the reported measure of the refraction, resulting in different estimates of the proportion of persons classified as myopic (and emmetropic and hyperopic). In some cases, the “errors” so induced may not have been unbiased. In addition, there were, no doubt, actual errors and statistical variation around the measurements that were, in general, not reported at all. Despite these concerns, some common findings emerged and some directions for further research have been provided as well. Because the thrust of this volume is on myopia, a refractive state, ocular biometry will not be discussed, although myopia results anatomically and physiologically from the biometry.

Review of Studies (Table 1)

For the sake of space and because of the possibility that there have been temporal changes in the distribution of refraction, only the more recent publications have been reviewed for the purpose of this chapter. Epidemiologic studies of refraction have burgeoned and now there are data from studies across the age span and in many different ethnic and cultural groups. For the sake of brevity, a limited amount of descriptive material from each study has been included; however, interested readers could refer to the reference citations. Still, this review is not exhaustive but is merely a (non-systematic) sample of studies of myopia.

Klein .K.E.B 6

Table 1. Overview of Studies

(Continued)

8B.E.K. Klein

The Sydney Myopia Study evaluated refraction in a sample of 6- to 7-year-old school children.8 The mean spherical equivalent refraction in these children was +1.26 D in the right eyes. The boys were slightly more likely to be myopic than the girls, and white children were slightly less likely to be myopic than non-whites. In the 12-year-old children in that study, spherical equivalent was less positive than in the younger children.9

Investigators from six sites in the U.S. pooled their data on refractive errors and ocular biometry in school children ages 6 to 14+ years.10 The students were from different ethnic backgrounds. They found no difference in average spherical equivalent between girls and boys; there was a shift towards myopia with increasing age in both.

There were 663 subjects who were 15-years of age or younger in the Andhra Pradesh Eye Disease Study11 (Table 1). Myopia was less common in those 15-years of age and younger (about 4%) than in older persons (19%). The first reported myopes were about 5-years-old.

The prevalence of myopia in Chinese school children 7–9-years of age in Singapore was 29%, with successively higher prevalences with increasing age.12 The age when the tendency for increasing myopia levels off is uncertain. Studies of university students suggest that this may continue into the third decade, although there may be the confounding effect of near work activities in these subjects.13,14

The Beaver Dam Eye Study reported that 28.1% of women and 24.0% of men 43–86-years of age were myopic. A difference between men and women was true throughout the age range.15 Overall, the prevalence was 26.2%. Wu reported that the prevalence of myopia decreased with age until 60 years but increased thereafter in a population of AfroCaribbeans.16 The Visual Impairment Project conducted in Victoria, Australia, included urban and rural persons.17 Prevalence decreased with increasing age; the overall prevalence was 17%. Overall, sex was not associated with myopia, but after age correction, women were slightly more likely to have a hyperopic refractive error. The Blue Mountains Eye Study is a population-based study of 3654 persons, 49–97-years of age.18 The overall estimate of prevalent myopia was 14%. The Baltimore Eye Survey reported on refraction in 2200 African Americans and 2659 white Americans in Baltimore.19 Overall, 25% of the population was myopic and whites had higher myopia on average than blacks. Myopia declined with increasing age. The Los Angeles Latino Eye Study found that in 5588 adults 40-years of age or older, the mean spherical equivalent was 0.02 D (+/−1.66) in men and 0.18 D (+/−) in women,20 and that on average there was 0.04 D